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SN74LVC8T245 lowest startup supply voltage and threshold.

Randy Wang
Intellectual 1705 points
Other Parts Discussed in Thread: SN74LVC8T245, SN74AVC8T245

Hi Team

  This is South China AFAE Randy who support Zhuzhou (China high speed train design), my customer is using many logic devices like SN74LVC8T245, and recently we need to review the design for initial status for each device.

  8T245 is started at 1.65V with high level voltage at 1.07V, so I’m wondering what’s the status below 1.65V? If I give 8T245 a 1.2V supply voltage with 0.65*1.2V input signal, does 8T245 output a high level voltage?

  Or can you tell me the lowest startup voltage for this device? Is the high level threshold also 0.65*Vcc at this moment? Thanks. 

  • 0
    TI__Expert 6335 points

    Randy,

    We only guarantee device performance under the operating conditions listed in the datasheet. If 1.2V is applied on either VCCA or VCCB, we can't guarantee the VIH limits that are listed in the datasheet.

    We do have a SN74AVC8T245 that supports a 1.2V VCC (with VIH >= 0.65*VCCI @ 1.2V). However, it does not support 5.5V VCC like the SN74LVC8T245.

    Can you give me a list of the customer's device requirements? From there, we can work to find the appropriate translator.

    Best Regards,
    Nirav

  • 0 in reply to Nirav Patel
    TI__Intellectual 1705 points
    Hi Nirav
    I'm not using these devices in such low voltage, just want to make sure the initial status, for example, if the device datasheet listed 1.2V Min startup voltage, and 0.65*VCCI threshold, we can has a resistor divider to make sure the input not trigger the threshold at the system startup, and work at like 3.3V supply voltage condition.
    So I think you may give some margin for this device, the device may startup at 1.0V or 0.9V, so I can give a resistor divider to make sure the input not trigger the threshold.
    Thanks.
  • 0 in reply to Randy Wang
    TI__Expert 6335 points
    Randy,

    I think I misunderstood your request, so let me see if I have it right this time:

    Is your concern during the power ramp of the SN74LVC8T245? As VCCA and VCCB are rising from 0V towards their steady state value, you want to ensure that the voltage on the inputs do not trigger any of the outputs to switch high, is that correct?

    Best Regards,
    Nirav
  • 0 in reply to Nirav Patel
    TI__Intellectual 1705 points

    Hi Nirav

      Yes, we're more concern about the startup behavior of SN74LVC8T245 as we don't know the exact voltage of SN74LVC8T245 start work.

      Or could you explain the startup sequence of this device? thanks. 

  • 0 in reply to Randy Wang
    TI__Expert 6335 points
    Randy,

    The threshold is proportional to the supply voltage. So it's possible for an output voltage to appear high if there is an input high during power up.

    My reccomendation would be to tie !OE to VCCA thorugh a pullup resistor. This way, as the SN74LVC8T245 powers up, Ports A and B are isolated until VCCA has reached steady state. Any input voltage would not be forwarded to the output during power ramp.

    Is this something that the customer can implement?

    Best Regards,
    Nirav
  • 0 in reply to Nirav Patel
    TI__Intellectual 1705 points
    Hi Nirav
    Thanks, I think this should be the best solution.